Magnetic resonance systems acquire data using strong magnets for providing large static magnetic fields. Gradient coils within the magnet are provided to "focus" the magnetic fields. The gradient coils and the large static magnetic fields are used to magnetically align certain nuclei ("spins") in a desired plane of the sample being imaged or spectrographically studied. A radio frequency (RF) pulse is used to "tip" the aligned spins so that at least a projection of the tipped spins is in a plane orthogonal to the plane in which the spins are aligned. When the RF pulse terminates the nutated or tipped spins dephase and tend to return to the aligned condition. As the spins return to the aligned state the movement of the spins in the magnetic field generate what are known as "free induction decay" (FID) signals. It is the FID signals in one form or another that are used for imaging and spectrographic purposes.
Special RF coils or probes are used for transmitting RF pulses and/or receiving the FID signals. These probes are energized in the transmitting state with an RF pulse frequency known as the Larmor frequency which, as is well-known, is a function of the particular element and the strength of the magnetic field in which the element is located. The Larmor frequency is also the precessional angular frequency of the aligned nuclei (spins) and the frequency of the (FID) signals.
While many types of magnets can be used to generate the large static magnetic fields; in a preferred embodiment a superconducting magnet is used. The subject or patient is placed in the bore of the superconducting magnet for exposure to the large static magnetic field. The RF probes are either body probes, where the patient fits inside the probe or probes designed to be juxtaposed to particular portions of the body such as the spine, the limbs or the head.
The probes must be capable of:
resonating at the desired radio frequency; PA1 generating homogeneous magnetic fields if used in the transmitting mode, and PA1 not adding excessive noise to the signals received. PA1 first coil means generating a circular magnetic field, PA1 second coil means generating a linear magnetic field in quadrature to said circular magnetic field, and PA1 said first and said second coil means being positioned relative to one another and being connected to provide circular polarization. PA1 a first of said two separate coil means comprising: PA1 a center conductor and two outer conductors, PA1 the center conductor and the two outer conductors being joined together by two spaced apart conductors joining the center conductor to each of the outer conductors at spaced apart points on each of the said center conductor and outer conductors, PA1 the second of the said two separate coil means comprising: PA1 a loop including conductor means serially coupled through circuit elements, PA1 the first and second coil means being substantially parallel and juxtaposed to each other, PA1 means for connecting the first and second coil means to provide an induced field that rotates relative to a part of the subject being imaged whereby a polarized RF probe is provided that is ideal for imaging of spinal columns since the center conductor can be aligned parallel and proximate to the spinal column and the other conductors of each of the two coils will also be relatively proximate to the spinal column.
The probes designed to be juxtaposed to particular portions of the body are relatively efficient due to the proximity of the probe to the body part from which data is acquired.
Notwithstanding this relative efficiency of the proximate probes (including surface coils), the signal-to-noise ratio (SNR) of the acquired data remain critical because of the small amplitudes of the FID signals. The SNR decreases because, among other things, of imbalances in the surface coils due to stray capacitance and because of variations in the impedances of the probes when "loaded" by the patient. Different patients have different body impedances and, therefore, load the RF probes differently. Also, the human body is not symetrical - thus the loading is not symetrical and results in variations in the signals received from the probe.
Another serious problem faced by the scientists and the designers of MR systems is that the RF power transmitted by the probes may cause heating of the body sections being studied. The heating occurs because only a relatively small portion of the RF power tips the spin; most of the power generates eddy and dielectric currents in the tissue of the subject which in turn generate heat. This RF heating has caused the Federal Drug Administration (FDA) in the United States to set a limit on the specific power absorption rate (SAR) of the RF signal that can be used in imaging humans. The set limit is 0.4 watts per kilogram. Thus there is a limit on the power that can be used by RF probes that is a function of the patient's weight. The limit is designed to safeguard the patient from exposure to RF caused heat damage to tissues.
Most of the probes used in the past have been linearly polarized. For example, "saddle" shaped coils have been extensively used. Linearily polarized as used herein means that the fields provided by the probes are normal and remain normal to one of the planes defined by two of the orthogonal axes of the MR system. Generally, speaking the MR system is considered as an XYZ orthogonal systems with the large static magnetic field in the Z direction.
The RF field is assumed to be perpendicular to the XY plane. The spins precess around the Z axis for example, and the effective projection and linear polarization is in the XY plane.
When using linear polarization of the applied RF pulses only half of the RF power of the generated magnetic lines pass through the subject. Accordingly, only half of the RF power is effectively used, at best, to tip the spins.
Still another problem is that the presently available RF probes cause what are known as radio frequency penetration artifacts which appear on the body images as shaded areas. The artifacts result from standing waves of the RF radiation passing through the tissue at high frequencies which distort the uniformity of the applied radio frequency magnetic field. In an attempt to overcome this problem the prior art implemented an excitation mode wherein the polarization is "circular" rather than linear.
See an article entitled "An Efficient Highly Homogeneous RF Coil for Whole Body Imaging at 1.5T" by C. E. HAYES et al in the Journal of Magnetic Resonance, Vol. 163, pages 622-628 (1985). This mode is also sometimes referred to as a "Quadrature Mode". The circular polarization in addition to improving image quality, reduces the power required to achieve a given shift of the spins. The circular polarization decreases the necessary RF power by a factor of 2. Accordingly, smaller RF power amplifiers can be used.
Also as a synergistic benefit; less energy is absorbed by the patient; thereby reducing the problem of possibly exceeding the 0.4 Watts per Kg. SAR. The sensitivity of the receiver coils to the FID signals are also greater with circular polarization by an amount that increases the signal to noise ratio by a factor of the square root of 2.
However, there has been a serious drawback involved in the use of circular or quadrature mode equipment in that the homogenity of the generated RF field in the subject does not match the homogenity of the fields generated by the saddle coils.
A related problem with quadrature mode equipment has been the difficulty encountered in providing coils which can generate circularly polarized RF fields without being unduly influenced by the loading by the patient. Also, quadrature mode generating equipment is generally unduly influenced by the cross-coupling between the multiple coils that must be used to generate the circular polarization.
The prior art attempts at accomplishing circular polarization or quadrature excitation have been accomplished using multiple spaced-apart coils. The multiple spaced apart coils comprise either two coils at 90 degrees to each other, counter rotating current resonators, planar pair resonators or extremely complicated quadrature surface coils. See for example an article entitled "Quadrature Detection Surface Coils" by James S. Hyde et al, which appeared in the Journal of Magnetic Resonance in Medicine, Vol. 4, pages 179-184 (1987). See also an article entitled "Quadrature Detection in the Laboratory Frame" by D. I. Hoult et al, published in the Journal of Magnetic Resonance in Medicine, Vol. 1, pages 339-351 (1984) which teaches a plurality of overlapping saddle coils which theoretically can be used in quadrature. Implementation of the theory has proven extremely difficult.
Another problem related to the use of quadrature surface coils is that none of the known quadrature surface coils can be effectively used for imaging the spine. For example, when two separate coils, 90 degrees to each other, are used then only one of the coils can be placed proximate to the spine while the other coil is kept away from the spine by the subject's body. The distance of the subject's body in effect makes the second coil irrelevant to the imaging process. Thus, none of the presently available planar pair of resonators, the counter-rotating current resonators, or the quadrature surface coils are conducive to use in spinal imaging.
Accordingly scientists in the field are still seeking efficient quadrature surface probes for use in MR systems and especially for such probes which can be used for spinal imaging.